Chemical Formulas

1
Chemical Formulas Chemical Compounds

• Chapter 7

2
Chemical formulas

• Common names give no info about chemical
  composition.
• Chemists use systematic methods for naming
  compounds writing formulas.

3
A chemical formula indicates the number of atoms
of each kind in a chemical compound.

• C8H18
• indicates there are indicates there are
• 8 carbons 18 hydrogen

4
Monatomic Ions

• Ions formed from a single ion
• Formed by gaining or losing electrons

5
Contd

• Cations
• Mg
• Magnesium
• Mg2
• Magnesium cation
• Anions
• O
• Oxygen
• O-2
• Oxide

• Monatomic cations are identified by the elements
  name.
• Monatomic anions are named by changing the
  elements ending to
• ide.

6
Binary compounds

• Compounds composed of two different elements
• Total number of positive negative charges must
  be equal
• The positive negative charges can be crossed
  over

7
Rules for binary compounds

1. Write the symbols for the ions side by side,
   using the cation first.
2. Cross over the charges.
3. Write the formula without the charges.

8
Problem

• Write the formula for the binary ionic compounds
  formed between the following elements
• magnesium iodine
• magnesium sulfur
• magnesium nitrogen

9
Nomenclature

• Nomenclature refers to the naming system.
• To name binary ionic compounds, combine names of
  positive negative ions.
• Al2O3 Aluminum
• Oxide
• name of name of
• cation anion changing
• ending to -ide

10
Stock system

• With some elements (mainly the transition
  elements), such as iron, 2 or more cations with
  different charges are formed
• Uses a Roman numeral to indicate an ions charge
• Numeral is enclosed in parentheses placed
  immediately after the metal name
• CuCl2 Copper (II) chloride
• name of cation name of
• Roman anion with
• numeral for -ide for
• charge ending

11
Oxyanions

• Oxyanions are polyatomic ions containing oxygen.
• The most common oxyanions end in ate.
• NO3- nitrate
• NO2- nitrite

12
Contd

• Sometimes more than 2 oxyanions are formed
• ClO4- perchlorate
• ClO3- chlorate
• ClO2- chlorite
• ClO- hypochlorite

13
Prefix system

• If only one atom of first element in formula,
  then no prefix on first element
• If more than one atom of the first element, the
  first name begins with prefix
• Second name begins with a prefix, followed by the
  root name, ending in ide
• P4O10
• tetraphosphorus decaoxide

14
Problem

• Give the name for As2O5.
• Write the formula for oxygen difluoride.

15
Acids Salts

• Binary acid consists of hydrogen halogen
• Oxyacid contains hydrogen, oxygen, a nonmetal
• Salt ionic compound composed of a cation the
  anion from an acid
• Table 7-5 on Page 214 Common acids!

16
Chemical formulas can be used to calculate the
formula mass, molar mass, percentage
composition.
17
(1) Formula mass (molar mass)

• The sum of the average atomic masses of all the
  atoms represented in its formula
• PROBLEM
• Find the formula mass of potassium chlorate.

18
(2) Percentage composition

• The percentage by mass of each element in a
  compound
• composition mass of element x 100
• molar mass
• PROBLEM
• Find the percentage composition of copper (II)
  sulfide.

19
Empirical formula

• By using the percentage composition, a
  substances empirical formula can be determined
• Shows the smallest whole number ratio of the
  different atoms in a compound

20
Rules

1. Change percentage composition to grams.
2. Divide grams of each element by its atomic
   weight.
3. Change to lowest whole number ratio by dividing
   each by the smallest number.

21
Problem

• Quantitative analysis shows that a compound
  contains 32.38 Na, 22.65 S, 44.99 O. Find
  the empirical formula of this compound.

22
Molecular formula

• Any multiple of an empirical formula
• An empirical formula may or may not be a correct
  molecular formula
• To determine the molecular formula, you must know
  the compounds molar mass

23
Problem

• The empirical formula of a compound of phosphorus
  oxygen was found to be P2O5. Experimentation
  shows that the molar mass of this compound is
  283.89 g/mol. What is the compounds molecular
  formula?